Summary of Work: Somatic and germinal mutations can have severe impact on the fitness of multicellular organisms and offspring. The ability to study the occurrence of mutations in higher organisms has been limited by available methods rendering it difficult to address important questions such a 1) kinetics of mutation induction in relation to organ specificity, 2) mutation induction during development, 3) systemic effect of somatic mutations of age-related degenerative diseases, and 4) tumor progression from pre-neoplastic to neoplastic growth in relation to genomic stability. Initially, PhiX174 with the well characterized am3 mutation is used as a transgene to evaluate substitutions at the A:T base pair. The animals for mutagenesis studies were produced by mating homozygotic am54 males to C57BL6/J females. Hemizygous male offspring from this cross were injected i.p. with 150 mg ENU per kg and were euthanized at various post-treatment intervals. The spontaneous revertant frequency in the spleen was 1.42 x per million plaque forming unit (pfu) and in the testis it was 1.41 x per million pfu's. There was no significant difference between the two tissues. In spleen, it was not until 110 days after treatment that the average revertant frequency among treated animals was significantly higher than the revertant frequency among the control animals. In testis the average revertant frequency 110 days post ENU injection was not significantly different from the control. However, two important observations were made regarding the testis data. First, two animals had an increased revertant frequency 110 days after ENU treatment while the other three animals in the group had revertant frequencies significantly lower than the average control frequency. Second, the variation in the revertant frequencies in the testis among the treated animals increased as the post injection period increased. Taken together, these two observations may indicate that the revertants formed clusters in individual testis samples. In the spleen similar discrepancies between revertant frequencies in individual samples were observed 3 and 10 days after treatment. At 110 days post-injection after treatment the revertant frequencies in individual samples converge towards an average.